Off-center squirming in viscous flow confined by a fluid pocket

Helicobacter pylori swims through gastric mucus by locally neutralizing its acidic environment and turning surrounding gel into a fluid pocket. The size and shape of the degelled pocket strongly affects swimming speeds in such confined environments.  To study the effect of three-dimensional confinement, we develop an exact analytical solution for a spherical squirmer confined in a spherical Newtonian fluid domain with no-slip boundaries. The squirmer can be placed anywhere along the centerline of the confinement, allowing us to study off-center motility and close-range hydrodynamic interactions of the swimmer and pocket walls. We report the swimming speed and flow field for different confinement sizes and swimmer locations, for radial and tangential pusher and puller squirmers. Tangential and radial squirmers have reduced and enhanced swimming speeds, respectively, when they are located at the center of the confining pocket. Depending on the squirmer strength and pocket sizes, tangential squirmers can have enhanced swimming speeds when off-center and near a wall, and swimming speeds can even reverse. We explain our results in terms of the interaction of swimmer flow fields and the confinement boundaries.